Dynamic Processes in Be Star Atmospheres. V. Helium Line Emissions from the Outer Atmosphere of λ Eridani

Abstract

The He I lines of the mild B2e star λ Eri often exhibit rapid, small-amplitude emissions that can occur at random places in their photospheric lines, even when the star is in a nonemission state. New simultaneous observations of the triplet λ5876 and singlet λ6678 lines show that the emission ratio for these lines is near unity, contrary to the predictions of either non-LTE model or nebular theory. Several He I emission events point to the formation of short-lived structures near the star's surface. On 1995 September 12 the line λ6678 exhibited a strong (0.13Icont) emission lasting some 20 minutes. The rapid decay of this feature implies a density of ≥1011.5 cm-3 for an emitting plasma structure near the star. This value is consistent with density estimates for slabs which may be responsible for ephemeral dimples in this star's He I lines on other occasions. We argue that photospheric helium emissions during Hα-quiescent phases are caused by foreground material and ask what mechanism might produce these features against the stellar background. To answer this question we have simulated He I line emission from model slabs having various properties and suspended over the star. We find that illumination by a source of extreme-ultraviolet (EUV) or X-ray flux depletes the He I column density so that it is difficult to form observable He I lines. A more interesting set of conditions occur for slabs with high densities (~1012 cm-3) and moderately large optical thicknesses in optical He I lines. Under these modified assumptions modest amounts of emission can be reproduced in singlet and triplet lines, and in the observed ratio. The key to producing this emission is for the slab to feel its own Lyman continuum radiation. This condition causes λ584 and other resonance lines to partially depopulate the ground state and to overpopulate the first few excited levels, ensuring that the departure coefficients of relevant atomic levels approach common values. The second necessary ingredient is a high density, which tends to equalize the departure coefficients of excited levels through recombinations and through redistribution of electrons among the l-sublevels. The combination is a kind of Lyman-pumped recombination because it relies on the Lyman continuum being marginally optically thick. Our results are consistent with studies of He I emission from planetary nebulae, symbiotic variables, and active galactic nuclei (AGNs), and may have a bearing on other detached atmospheres problems as well. This study appears to be the first application of such a mechanism to a quasi-photospheric setting.